This invention relates to time measurement and is particularly concerned with the transposition of a time measurement from one time frame to another as may be required in a process monitoring and control system carried out by means of a stored program, digital data acquisition and control system which utilizes a master station and a number of remote stations. More particularly, this invention relates to a method for accurately determining the time between events at locations remote from each other or for ordering the time sequence in which the events occur when there are no synchronized clocks available at the separate locations for accurately time tagging the events in accordance with a single time frame.
In the past, the correlation or measurement of time to an accuracy of a few microseconds has required expensive atomic clocks. In addition to the expense of the atomic clocks there is the added expense of maintaining their accuracy by using either an expansive periodic setting service which involves portable standards or the use of the television line 10 synch pulses as a means for setting the clocks.
The use of line 10 synch pulses is fully described in NBS TECHNICAL NOTE 695 published May 1977. That method consists of noting the clocks reading at predetermined times of day. Then at a later date when the NBS publishes the times for those lines as determined from their standard, the clock being checked can be adjusted to take into account the errors. These time corrections are, however, not made in real time as is the method of the present invention. In addition the NBS system does not provide real time updating as is needed for power systems such as those with which this invention is concerned. This method also requires the use of a gross standard such as WWVB.
Other systems have used the WWVB time standard which because of its low bandwidth gives accuracies of about 1 ms.
Still others systems have utilized measurements at both a master location and at remote locations which refer all time measurements to the master clock by indirectly measuring transit time to the remote locations and back, as is fully described by coworkers of mine in U.S. patent application Ser. Nos. 301,349 now abandoned and 301,350 filed Sept. 11, 1981 now U.S. Pat. No. 4,473,889.
Since the most important aspect of the problem is the accuracy of the timing of each event relative to a private master clock and not the exact time of day relative to a master clock at Greenwich or to the earth's rotation, highly accurate clocks are not necessarily a necessity. It is, therefore, desirable to eliminate the expense and complexity of precision clocks at each remote unit while maintaining high overall accuracy of correlation. Thus, it is an object of this invention to use medium accuracy clocks at both master and remote locations in a way which will provide highly accurate measurments of time differences between events at different locations or for the time sequential ordering of such events with compensation for the invariant differences in time as read by the clocks as well as for the different rates at which the clocks drift with time.
It is also an object of this invention to make possible simplified measurements of voltage phase angles in a power transmission and distribution systems with respect to a common swing bus by making highly accurate measurements of the zero crossings of the voltage wave form at various buses in the system.
Also, it is an object of this invention to make possible the simplified measurement of the location of a fault by simply measuring the time at which a wave front representing the fault current arrives at the two ends of the transmission line in which the fault has occurred.